This blog discusses current weather, weather prediction, climate issues, and other topics

Saturday, July 15, 2017

How Western Washington's Temperatures are Controlled By the Pacific Ocean

Have you ever thought about how boring and relatively unchanging our high and low temperatures are? Particularly, when we are in our normal situation with modest onshore (westerly) flow?

To illustrate, here are the temperatures at Seattle Tacoma Airport for the last two weeks (red lines), with the typical highs and lows. We had a warm spell (6th and 7th, highs around 85F) when we had a bit of offshore flow, but most days reach their max between 70 and 80F, with the long-term average in the low to mid 70s. The low temperatures are MUCH more uniform--roughly 53-58F.

In contrast, a location like New York City, also on a coast, has huge variations, with the low temperatures ranging from 80F to the upper 60s. So why are we different?

The answer is the Pacific Ocean and the typical onshore flow at low levels during our summer.

During the summer, high pressure is typically resident over the eastern Pacific Ocean, with lower pressure inland, resulting in gentle onshore flow at low levels (see example for Monday, July 10 at 5 AM).

So the air reaching Seattle and western Washington had been over the Pacific Ocean for a long time, with the air near the surface taking on the temperatures of the ocean surface.

So what are the ocean temperatures off our coast right now? Here they are (below). Roughly 14-16C (57-61F), with some colder water from upwelling (water coming up from below) right near the coast, and a bit colder water offshore. So roughly the upper 50s. This is the kind of air that is flooding our region at night, without solar radiation to warm it up. During our short (and generally clear) nights, there is radiational cooling from the surface, so that can take off a few degrees.

Not only are our temperatures relatively uniform, but so is the humidity or moisture content. Our air can't pick up much moisture over the water because it is relatively cold...a very different story than that warm Gulf of Mexico. Thus, our dew points (the temperature you must cool air to get saturation) are typically no more than the lower 50sF during the summer. That is important, because high dew points can keep the temperatures up at night (like in the humid East Coast).

So there is no real mystery what our temperatures typically drop into mid to upper 50s at night...and do so in a very reliable way. It is the Pacific and its modest temperatures.

But why do we have highs in the 70s during the day when we have onshore flow and relatively clear skies, like the last few weeks?

Yes, it is the Pacific again. We start off with a water temperature of say 57F (14C), with the nearby air of similar temperature. Over the ocean, without surface heating, the lower atmosphere is relatively stable, with only a small change of temperature with height. With high pressure dominating the eastern Pacific, there is sinking air in the middle to lower troposphere, which weakens towards the surface. Such sinking tends to produce stable conditions and often inversions (temperatures increasing with height).

The temperature sounding Friday morning at Quillayute on the Washington Coast illustrates this. Red line is temperature and heights are in pressure (700 is about 10,000 ft). Click on the image to expand.

The typical lapse rate (change of temperature with height) along the coast at the radiosonde site at Quillayute during the summer is roughly 3.5C per km (the graph is from the sounding climatology page of the NOAA Storm Prediction Center). The black line are the average values.

So if the temperature at surface is 15C, the temperature at 1.5 km (about 850 hPa) is roughly 5.25 C less. Let's call it 10C. That air blows in over Seattle and western WA.

Now why do I care about 1.5 km and 850 hPa?

Because during the day over land, surface heating causes the air to mix in the vertical and it usually reaches that height. That mixing is associated with a lapse rate (the adiabatic lapse rate) of 9.8C per km. To put it another way, air coming in over the ocean can be mixed down to the surface, where it is warmed by compression by 9.8C per km. Or to say it differently, the air is warmed by roughly 15C as it descends 1.5 km to the surface. If the air started at 10C, it would be 25C at sea level. 25C is 77 F! Our typical high temperature during the middle of the summer.

I know this explanation is a bit complicated.

But the bottom line is that we have the sea surface temperature offshore driving the surface air temperature over the ocean. That is connected with temperatures of air in the lower atmosphere that moves inland, which can mix to the surface during the day. The result is a direct connection between the temperature of the sea surface of the eastern Pacific and our high temperatures. At night, when vertical mixing is minimal the connection is more direct, with low level air just moving in horizontally.

Offshore flow changes everything... our temperatures become disconnected from the slowly changing ocean temperatures, and we can get much more extreme temperature variation. So thank the ocean for our perfect summer temperatures. And it is this connection with the ocean that will slow the impacts of global warming over our region, giving us more time to deal with that issue.

9 comments:

I'm curious: Does the Pacific have that same effect all the way down the coast of the Americas -- all the way through Patagonia?

My husband and I were just wondering yesterday, actually, if the predictions about global warming's effects would be similar in the PNW and in Patagonia, since they're at similar latitudes and both on the Pacific Ocean.

Always knew the winds off the water kept us cool and understood lapse rates (used to be a climber in the Rockies), but it's interesting to learn the machinery of mixing behind my simplistic understanding.

Your use of the word Easterly, meaning winds moving to the east, tickled my fancy. That, and Westerly, are one of the English language's most maddening words.

1) Easterly, used by itself, means winds blowing west to east.

2) Easterly Winds means the opposite: winds blowing from east to west.

3) Easterlies are also from east blowing west.

Even Dictionary.com says: "Easterlies definition, moving, directed, or situated toward the east: an easterly course." But that's only valid if it DOESN'T refer to wind.

According to NOAA: "Easterlies: Any winds with components from the east."

Even stranger, "winds are easterly" means the exact opposite of "easterly winds".

Anyway, I'm a bit off topic, but this is a wonderful source of mass confusion for most people.

Without a doubt, we have the best summer weather in the lower 48. Escaping the summer 70F+ dewpoints from back east is the principal reason I moved here decades ago. It was one of the smartest decisions I've ever made.

When I first moved here from the Dakotas in 97 I could't believe these summers. There is this....persistent cool breeze from the west, and it's making the outdoor work comfortable?! Now in order to appreciate my reflexes one must know the dakotas are prone to sticky, humid summers. To summarize. If I ever retire it will be winters in Tucson and summers in the Pac NW

Actually, John K, easterly it can be an adv, adj or noun. Here's an example from Vocabulary.com on the adverb form:

-------------adv: from the east“the winds blew easterly all night”-------------

So it's not a grammatical issue, but rather a definition issue.

And in this example, easterly is used to define an off-shore wind (from the east), yet in Cliff's usage, easterly means on-shore, west to east.

This may sound like a grammatical exercise, but when you're cruising on a sailboat, and you get these meanings confused, you could find yourself being forced up onto a lee shore that you thought was going to be the weather shore.